In a known apparatus of this kind, the stepping motor is operatively connected to the axially displaceable control rod by a rack and pinion drive so that the backlash between the stepping motor and the control rod adversely affects the control particularly during operation under no load. Another important disadvantage which often arises is that the means for transmitting force from the stepping motor to the control rod do not include a resilient member so that the stepping motor must be very powerful and large because very high frictional forces must be overcome during the injection of fuel. If a stepping motor is used which is less powerful and cannot adjust the control rod during the injection of fuel, additional signalling means are required to indicate that no adjustment has been effected.
In another apparatus, which is known from European Patent Publication No. 0 069 111, a mechanical energy storage device is connected between the control rod, the associated actuating motor consisting, e.g., of a stepping motor and that energy storage device comprises two spring discs, which are movable relative to each other, and a prestressed compression spring disposed between said spring discs, the maximum separation of which is defined by stops in a housing which embraces the two spring discs. In that case too, a rack-and-pinion drive, which involves a backlash, is connected between the actuating motor, e.g., the stepping motor, and the linkage which includes the energy storage device, and a backlash is likely to occur also in the linkage.